| args: argv[2] = 3
| >factorial
| | find: find 3 factorial
| | >factorial
| | | find: find 2 factorial
| | | >factorial
| | | | find: find 1 factorial
| | | | result: result is 1
| | | <factorial
| | | result: result is 2
| | <factorial
| | result: result is 6
| <factorial
6
<main
